An important parameter in many control systems is temperature. One of the most commonly employed mechanisms for dealing with the control of temperature is the thermocouple sensor. Thermocouple sensors are utilized to measure the temperature in high temperature environments such as those associated with autoclaves, furnaces, boilers, etc. Consequently, the prior art is replete with patents describing thermocouple devices of various configurations and constructions.
The Type K thermocouple sensor (Ni/10 Cr versus Ni/5 (Si, AI)) is presently employed in a wide array of temperature measurement and control applications. As stated earlier, the thermocouple sensor is coupled to the instrumentation by way of an extension cable. It is necessary that the thermal EMF of the extension cable is the same as the thermocouple sensor from 0.degree. C. to the temperature of the transition where the extension cable is connected to the thermocouple sensor. It is desirable, from the standpoint of maintaining accuracy of measurement, for the thermocouple extension cable to exhibit the lowest possible loop resistance. Lowering the loop resistance of an extension cable allows the same instrument error limits with extended lengths of the extension cable. This is an advantage in applications where very long distances on the order of 100 feet or more exist between the thermocouple sensor and the instrumentation. For example, very long extension cables are employed between thermocouple sensors used in oil fields and the requisite instrumentation. These cables can be on the order of 100 feet or longer. Thus, in this application, a cable having lower loop resistance would greatly increase the accuracy of the temperature measurements.
Further, an extension wire that has a lower loop resistivity value allows the use of a smaller diameter wire for a given length of cable. Reducing the cable diameters also provides the benefit of enhanced cable flexibility.
Two standards setting forth the initial accuracy requirements for thermocouple sensor extension wire are maintained in the industry, one being the U.S. standard and the other being the international standard. The U.S. standard tolerance (established by ANSI, ISA, NIST, and others) for Type K extension wire (KX) is .+-.2.2.degree. C. The IEC international standard tolerance for Type KX is .+-.2.5.degree. C. In the U.S. standard, only type K thermocouple alloy is used as KX extension wire. The applicable temperature range for KX wire, both under the U.S. and the international standard is 0.degree. to 200.degree. C.
Most thermocouple extension cables are insulated with a low temperature material such as Poly Vinyl Chloride (PVC). The inventors herein have, therefore, recognized that PVC insulated KX cables provide an effective operating temperature well below 200.degree. C. (The operating temperature of a PVC insulated KX cable is limited by the PVC insulation which has a maximum operating temperature 105.degree. C. as established by Underwriters Laboratory [UL]). The consequences of all this is that the users are paying for unneeded accuracy above 105.degree. C.
Hence, it becomes apparent from this disclosure that a switch to an extension cable manufactured from a metal costing less than KX, which meets the industry's initial accuracy requirements for thermocouple sensor extension cables up to 105.degree. C., would result in a substantial cost savings to users of the cables. Moreover, if this metal also exhibits a lower loop resistivity and thus a lower loop resistance, this would allow the use of a smaller diameter wire to achieve additional material savings.
It is, therefore an object of the present invention to provide an alloy composition for use in the manufacture of thermocouple extension cables having a lower loop resistivity and lower material cost than presently available compositions used in the manufacture of KX extension cables for use up to 105.degree. C.